The oxidation of alcohols to afford carbonyl compounds is a key reaction in synthetic organic chemistry. Recent years have seen major progress in the development of catalytic aerobic alcohol oxidation, which offers economic and environmental benefits over traditional stoichiometric oxidants (Schultz, M. J.; Sigman, M. S. Tetrahedron 2006, 62, 8227-8241; Punniyamurthy, T.; Velusamy, S.; Iqbal, J. Chem. Rev. 2005, 105, 2329-2363; Stahl, S. S. Angew. Chem. Int. Ed. 2004, 43, 3400-3420). Despite these advances, many reported catalysts include precious metals (Pd, Ru, Ir) and use oxygen pressures of 1 atm or more (Schultz, M. J.; Adler, R. S.; Zierkiewicz, W.; Privalov, T.; Sigman, M. S. J. Am. Chem. Soc. 2005, 127, 8499-8507; Steinhoff, B. A.; Guzei, I. A.; Stahl, S. S. J. Am. Chem. Soc. 2004, 126, 11268-11278; ten Brink, G. J.; Arends, I. W. C. E.; Sheldon, R. A. Science 2000, 287, 1636-1639; Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S. J. Org. Chem. 1999, 64, 6750-6755; Johnston, E. V.; Karlsson, E. A.; Tran, L. H.; Akermark, B.; Backvall, J. E. Eur. J. Org. Chem. 2010, 1971-1976; Nikaidou, F.; Ushiyama, H.; Yamaguchi, K.; Yamashita, K.; Mizuno, N. J. Phys. Chem. C 2010, 114, 10873-10880; Mizoguchi, H.; Uchida, T.; Ishida, K.; Katsuki, T. Tetrahedron Lett. 2009, 50, 3432-3435; Wolfson, A.; Wuyts, S.; De Vos, D. E.; Vankelecom, I. F. J.; Jacobs, P. A. Tetrahedron Lett. 2002, 43, 8107-8110; Arita, S.; Koike, T.; Kayaki, Y.; Ikariya, T. Angew. Chem. Int. Ed. 2008, 47, 2447-2449; Jiang, B.; Feng, Y.; Ison, E. A. J. Am. Chem. Soc. 2008, 130, 14462-14464; Gabrielsson, A.; van Leeuwen, P.; Kaim, W. Chem. Commun. 2006, 4926-4927). There is considerable interest in replacing precious metal-containing catalysts with base metal-containing catalysts because base metals are less expensive and more abundant that precious metals (Michel, C.; Belanzoni, P.; Gamez, P.; Reedijk, J.; Baerends, E. J. Inorg. Chem. 2009, 48, 11909-11920; Jiang, N.; Ragauskas, A. J. J. Org. Chem. 2006, 71, 7087-7090; Gamez, P.; Arends, I. W. C. E.; Sheldon, R. A.; Reedijk, J. Adv. Synth. Catal. 2004, 346, 805-811; Gamez, P.; Arends, I. W. C. E.; Reedijk, J.; Sheldon, R. A. Chem. Commun. 2003, 2414-2415; Chaudhuri, P.; Hess, M.; Florke, U.; Wieghardt, K. Angew. Chem. Int. Ed. 1998, 37, 2217-2220; Marko, I. E.; Giles, P. R.; Tsukazaki, M.; Brown, S. M.; Urch, C. J. Science 1996, 274, 2044-2046). Using air instead of pure oxygen (O2) is also advantageous (for selected examples of aerobic oxidation using air see: (a) Zahmakiran, M.; Akbayrak, S.; Kodaira, T.; Ozkar, S. Dalton Trans. 2010, 39, 7521-7527; Bailie, D. S.; Clendenning, G. M. A.; McNamee, L.; Muldoon, M. J. Chem. Commun. 2010, 46, 7238-7240; Conley, N. R.; Labios, L. A.; Pearson, D. M.; McCrory, C. C. L.; Waymouth, R. M. Organometallics 2007, 26, 5447-5453; Guan, B.; Xing, D.; Cai, G.; Wan, X.; Yu, N.; Fang, Z.; Yang, L.; Shi, Z. J. Am. Chem. Soc. 2005, 127, 18004-18005; Iwasawa, T.; Tokunaga, M.; Obora, Y.; Tsuji, Y. J. Am. Chem. Soc. 2004, 126, 6554-6555; Korovchenko, P.; Donze, C.; Gallezot, P.; Besson, M. Catal. Today 2007, 121, 13-21; Hara, T.; Ishikawa, M.; Sawada, J.; Ichikuni, N.; Shimazu, S. Green Chem. 2009, 11, 2034-2040), reducing the safety hazard associated with heating organic solvents under elevated O2 pressures.
Vanadium complexes have shown potential as base-metal catalysts for aerobic alcohol oxidation, in some cases proving effective for substrates where palladium catalysts display limited activity. For instance, vanadium is known to catalyze the selective aerobic oxidation of propargylic alcohols, a reaction using VIV(O)(acac)2 (1-5 mol %) and molecular sieves at 80° C. (Maeda, Y.; Kakiuchi, N.; Matsumura, S.; Nishimura, T.; Kawamura, T.; Uemura, S. J. Org. Chem. 2002, 67, 6718-6724; Maeda, Y.; Kakiuchi, N.; Matsumura, S.; Nishimura, T.; Uemura, S. Tetrahedron Lett. 2001, 42, 8877-8879). The combination of VIV(O)(acac)2 and DABCO (DABCO=1,4-diazabicyclo[2.2.2]octane) also catalyzes the oxidation of benzylic and allylic alcohols in ionic liquid solvent at 80-100° C. (Jiang, N.; Ragauskas, A. J. Tetrahedron Lett. 2007, 48, 273-276; Jiang, N.; Ragauskas, A. J. J. Org. Chem. 2007, 72, 7030-7033). Vanadium catalysts with chiral Schiff-base ligands effect the oxidative kinetic resolution of α-hydroxyesters, amides, and phosphonates (Radosevich, A. T.; Musich, C.; Toste, F. D. J. Am. Chem. Soc. 2005, 127, 1090-1091; Pawar, V. D.; Bettigeri, S.; Weng, S. S.; Kao, J. Q.; Chen, C. T. J. Am. Chem. Soc. 2006, 128, 6308-6309; Weng, S. S.; Shen, M. W.; Kao, J. Q.; Munot, Y. S.; Chen, C. T. Proc. Nat. Acad. Sci. 2006, 103, 3522-3527). These reports are promising indications of the versatility of vanadium catalysts, but each requires an atmosphere of pure oxygen. Very recently, Ohde and Limberg reported a metavanadate-cinnamic acid catalyst that catalyzes the oxidation of activated alcohols using mixtures of argon and O2, but this catalyst is highly moisture sensitive (Ohde, C.; Limberg, C. Chem. Eur. J. 2010, 16, 6892-6899).
There is a need in the art for an oxidation process that utilizes a robust base metal catalyst which can oxidize alcohols in the presence of air.